And I also expect other people to act in their own best interests. The current centralised systems promote psychopaths to rise to the top. A decentralised system based on free markets, the nap, and contract law removes the risk of such people having all the power, simply because there is no way to have all the power without the central control mechanisms.

1. If such a scenario existed, it is cheaper and easier to work within the framework rather than trying to subvert it.
2. My point here was a) the argument you made was a non sequitur. There are no areas that support a polycentric monetary system today, but it did exist and was more robust than the current system: https://www.sciencedirect.com/science/article/abs/pii/S0167268118302877. B) Also btc is not technically Fiat, it is closer to a limited resource by design. The others are less so, I'll grant you, but nothing stops people creating non-fit versions.
3. I do expect people to try. That's why there are systems to mitigate that based on market economics rather than force. As a warlord, you have to pay an army. You have to feed an army. You have to give that army soft targets. If there are hundreds of organised militias attacking you with the same equipment you have, the chance of your survival is infintessimal. If you have more powerful weapons, then tactics like America used in the revolutionary war apply. All it takes is a good sniper, or sufficient economic pressures, to turn a warlord into a corpse.